Aircraft hold-down device



July 9, 1946. H. F. PITCAIRN AIRCRAFT HOLD-DOWN DEVICE File July 17, 1941 4 Sheets-Sheet 1 July 9, 1946. H. F. PITCAIRN AIRCRAFT HOLD-DOWN DEVICE Filed July 17, 1941 4 Shegts-Sheet 2 July 9, 1946. H. F. PITCAIRN AIRCRAFT HOLD-DOWN DEVICE 4 Sheets-Sheet 3 Filed July 17, 1941 INVENTOR:

July 9, 1946. H. F. PITCAIRN AIRCRAFT HOLD-DOWN DEVICE 4 Sheets-Sheet 4 Filed July 17, 1941 V TOR: A444 7. m

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' after alanding has been made in order to hold Patented July 9, 1946 i V UNITED STATES PATENT OFFICE AIRCRAFT HOLD-DOWN DE ICE Harold F. Pitcairn, Bryn Athyn', Pa., assignor to Autogiro Company of America, Philadelphia, Pa., a corporation of Delaware Application July 17, 1941, Serial No. 402,769

14 Claims. (01. 244-110) 1 2 This invention relates to an aircraft hold-down erative to adjust the angle of the strut to subdevice, and is pa ly adapted to aircraft stantially parallel the-normal landing flight path.

such as those equipped with one or more sustain- In accordance with another aspect of the ining rotors, which craft are capable of substanvention, the hold-down strut is of multi-part contially vertical, or at least very steep, descent to 5 struction providing for extension and contraca landing. Features of the'invention ar further tion thereof. Automatic means are also conof use where the aircraft is also adapted for vertemplated for contracting the strut immediately ticaltake-oif. upon making a landing, so that the aircraft is Although the invention is not limited thereto, drawn down tightly to the landing platform. it is of especial importance and use for ship deck Still another feature is the provision of holdlandings and take-offs, where these operations down means independent of the hold-down strut and especially landing are complicated not only mentioned, such additional means being adapted by the restricted space available for the landing to hold the aircraft to the platform when driving platform, but also by the fact that the landing the rotor in preparation for take-off. A mechaplatform partakes of the pitching, rolling and nism is further provided for automatic release of heaving motions of the ship. the hold-down mean when the aircraft controls In accordance with one of its broader aspects, are actuated to eifect take-01f. 1 the invention contemplates the provision of co- How the foregoing and other objects and adoperating devices or elements on the aircraft vantages are attained will be apparent from the and'on the landing platform adapted tointerfollowing description, r ng to the a comengage to hold the aircraft tightly to the landin panying drawings, in whichplatform after a landing has been made, the said u e 1 is a s de t e V w O a aircraft devices or elements being so arranged as to be of e rotative'wing ype eq ipped with the mechefi cti in any one of a multiplicity of positions anism of the present invention, the aircraft here in which the aircraft may alight on the platform. eing shown on the landi g a i h the Preferably, the foregoing is accomplished by the hold-down strut m Operatlve Posltlon;

employment of a grillwork landing latform and Figure 2 i an enlarged vertical sectional view of a portion of the landing platform and of the by equipping the aircraft with a downwardly n hl-ownsruttheltr in meeting strut the lower emi Yvhlch 1s adapt" i n di fi ere t aigle t3 illustrat tl ie Bin ed to enter any one of a multiplicity of apertures t f 1 in the grillwork, the strut being provided with a 1 1 yo opera we ang means adapted to engage gnnwork element strut taken as indicated by the section line 33 on Figure 2;

t ii b i f resides in Figure 4 is a view taken generally in plan, parn 0 J c ticularly illustrating portions of the mechanism the mounting of the strut 1n the aircraft in such for mounting the Strut in the aircraft and for Figure 3 is a cross sectional View through the manner that it has freedom for movement to comacting and projecting the Strut. pensate for forward and sideward shifting of po- F1 gm e 5 is an enlarged View of certain parts sition of the aircraft on the platform during the 40 (appearing on a Smaller Scale in Figure 1) of a landmg operatlon' hold-down means adapted for use when preparing The means carried by the strut for mterlockfor takemfi; ing with the landing platform desirably take the Figure 6 is a diagrammatic View f a modim form of retractable barbs latches, and W cation of the hold-down strut for landing, moorsiOn is made for the to retract Said barbs porating automatic means for contracting the at a time when take-01f is desired. The invenstrut upon making a landing; tion further provides means for automatically re- Figure 7 is a, schematic iew of another odtracting the barbs when certain controls are actuification, incorporating means automatically ted o effect di fl s will further apter-relating release of the barbs (on the holdpear. down strut) with operation of the aircraft con- The invention also contemplates the employtrols for take-off; and ment of pilot operated means for projecting and Figures 8 and 9 are's'ectional views through retracting the hold-down strut so that, when in the control valve of Figure '7, showing that valve normal flight, the strut may be withdrawn within different positions.

in'the fuselage fairing, said means being also op- .Referringto the drawings, the invention is here shown as applied to an aircraft comprising a body 6 having an engine at the nose, enclosed in cowling l, the engine driving the propulsive airscrew 8, and further having a single sustaining rotor consisting of blades 9 pivoted to a rotative hub H] by means of flapping, drag and pitch pivots ll, l2 and [3. The sustaining rotor is mounted on the body by means of a pylon structure l4, a non-rotative part of the hub being pivotally connected with the pylon to provide freedom for tilting movement in all di-- rections for purposes of control. One such tilting pivot appears at l5, and a connection for control of tilting is shown, at lfias extending downwardly to the control stick IT. The control form.

As shown in Figure 2, the grillwork platform consists of a CT -SS-PCYQSS of interconnected eleper se, but such control system is of importance in an aircraft capable of substantially vertical descent and/or take-off, as is herein contemplated. A control system of the general type here involved is fully disclosed and claimed in copending application of Juan de la Cierva, Serial No. 645,985, filed December 6, 1932, which application issued July 31, 1945, as Patent No. 2,380,580.

The controls for the aircraft may also include a controllable rudder l8.

The landing gear includes a pair of forward wheels, one shown at H, and a tail wheel 20, the Wheels [9 being connected with the body by compressible shock struts 2| and diagonal struts 22.

Another feature of the general structure of the aircraft is the provision of means for effecting generally vertical take-lf. It is thus contemplated that the aircraft is of a type which may operate both to and from a small area landing platform, such as may readily be mounted on almost any ocean going vessel.

Although with the particular aircraft illustrated the sustaining rotor is aerodynamically or autorotationally actuated during normal flight, the invention is also applicable where the rotor is power driven during'normal flight. Where the rotor is normally aerodynamically actuated, provision may be made for substantially vertical take-off by driving the rotor from theforward propulsion engine with the rotor blades at zero or low pitch, whereby to store kinetic energy in the rotor which is subsequently released by raising the blade pitch to a substantial positive value, the rotor drive mechanism then being disconnected, for which purpose a manually operable clutch is used. The details of such jump takeoff mechanism need not be considered herein, although in Figure 1 there is shown rotor drive shafting at Illa and a gear and clutch unit at Iflb. There is also illustrated in Figure l a pitch change mounting l3 for each rotor blade 9 (only one being shown), the control of pitch being secured through a hydraulic system including piston and cylinder device 23 (which may be of the type disclosed in the Ray patent mentioned below), adapted to be supplied with fluid under pressure (to reduce the blade pitch) through piping:23a, later referred to. It may be mentioned that the. rotor drive desirably incorporates an overrunning clutch (not shown) to ensure free autorotation in normal flight.

For further particulars relating to jump takeoff mechanisms, reference may be made to Patent 2,216,163', issued October .1, 1940, of James G.

Ray, or to copending application of Juan de la Cierva, Serial No. 738,349, filed August 3, 1934,

lil

ments 25, although-any other appropriate pattern or disposition of the elements may be used. Each of the elements 25 desirably is tapered towardits upper edge, as indicated at 26, so as to cooperate with the hold-down strut in guiding the latter into an aperture in the grillwork reardless of the position in which the strut approaches the platform whenmaking a landing. The hold-down strut is indicated generally in Figure l at 21, the same being shown in this view in full lines in extended position and in dot. and dash lines (at 2711) in retracted position. The strut consists of a lower part 28 the upper end of which is adjustably telescoped with an upper part 29, which latter is mounted in the aircraft as will further appear, At its lower extremity, the part 23 is rounded or tapered off to a substantial point, as indicated at 39, so as to cooperate with the beveled edges 26 of the grillwork elements 25 in ensuring entrance of the strut into an aperture of the grillwork regardless of the position in which a landing is beingmade on the platform.

Toward its lower end the part 28 of the strut is provided with at least one, and preferably sev eral, vertically spaced pairs of releasable barbs or latches 3!. As seen in Figure 2, each pair of latches 3! is mounted on a common pivot pin 32, the strut member 28 being appropriately recessed so that when the latches are retracted they lie substantially. within the overall dimensions of the strut; This position of the latches 3! on pivot 32 is shown in full lines in Figure 2. lines, at 3 I a, a pair of latches is shown in extended position. At least'the' initial movement to the extended position is effected. under the influence of a leaf spring 33, the two ends of which bear respectively against the twolatches of a pair, the spring being deflected to U-shape when the latches are retracted into the strut, as clearly appears in Figure 2; Preferably the springs move the barbs substantially all the way out, so-that gravity need not be relied on.

Extension of the latches is limited by cooperating interengageable stops 34 which abut each other within the confines of the strut below the pivot 32, as shown. When the lower end of the strut enters an. aperture between elements of the grillwo-rk landing platform, the barbs will be forced together during passage of the strut downwardly and then will spring apart so as to engage under surfaces of grillwork elements. Because of the pivotal mounting of each pair of latches they may rock about the pivot 32 as a unit and thus automatically adjust themselves to tightly engage the under surface of the grillwork.

It will be observed from Figure 2 that the de-' gree of extension of the latches and the spacing of the elements 25 of the grillwork ensure that .re strut camiot. be withdrawn from the grill work aperture until the latches are retracted.

For the purpose of retracting the latches, fiexible In clotted cables 35, are connected therewith and extended upwardlythrough the strut to an operating ring or. other appropriate handle 36 which, asshown in Figure 1, is so located as to be readily accessible by the pilot. As seen in Figure 3, the cables 35 for the lower pair of barbs 3| .may pass the barbs of the upper pairs at opposite sides thereof suitable apertures being provided in the pivot pin 32 for this purpose. 1

Referring again to Figure 1, attention is now called to the fact that thelower member 28 of the strut may move vertically in the hollow upper member 29, a releasablelatch 31 being employed to normally retain member 28 in an intermediate position. Spring 38 acts-t,o maintain the latch 31 in engagement. After making a landing, when it is desired to move the aircraft around on the platform, an attendanton the platform may release latch 31, which will permit strut member 28 to drop and loosen the engagement of the latches, whereupon the pilot may then retract the latches 3i and draw strut ,member 28 upwardly by pulling on the operating member 36, thereby disengaging the strut from the aperture in the landing platform, so that the aircraft may be moved about as desired.

The upper end of strut member 29 is equipped with a spherical ,part 39 which is mounted in a spherical rubber cushion 49 which, in turn, is secured in casing 4|. The casing is apertured at top and bottom to pass member 29, the apertures being round and considerably larger than member 29 so as to permit swinging self-adjustment of the strut in any direction, and especially in a fore and aft plane, so that upon making a landing, when the strut enters an aperture in the landing platform, some movement of the aircraft, forwardly, rearwardly, or sidewardly will be permitted through the capability of the strut to move to diiferent angles because of the resilience of the rubber cushion 49.

As best seen in Figure 4, the mounting casing 4| for the strut is provided with trunnions 42 which are journaled in bearings 43 connected with fuselage framing, thereby providing a transverse swinging axis for the entire strut assembly, so that the strut may be retracted to the dotted line position 21a shown in Figure 1. Projection and retraction of the strut to and from the operative position is placed in the hands of the pilot, for which purpose a worm gear 44 is connected with one of the trunnion shafts 42, the gear cooperating with worm 45 mounted on shaft 46 which is connected, through bevel gearing 41, with an operating crank 48. The crank 48 is disposed for convenient access to the pilot.

With the mechanism described above, landings on small platforms on board ship may readily and safely be effected, for which purpose the pilot will, of course, operate the control crank or handle 48 to extend the strut to a position such as shown in Figure 1, and may then bring the machine down in a steep glide, or substantially vertically. As soon as the lower end of the strut enters an aperture in the grillwork landing platform, the latches 3| will spring out and thereby prevent the aircraft from being tossed about, notwithstanding pitching or rolling movement of the ship.

The provision of the retracting control enables the pilot not only to completely withdraw the strut during normal flight or when it is not desired to use the strut for landing, but also to adjust the angle of projection of'the strut to meet varying wind and other landing conditions. Since the strut is equippedwith several pairs of latches,- at different elevations, the aircraft will effectively be held down to the landing platform notwithstanding difierences in the adjusted position of the strut, and further notwithstanding differ: ences in the loading of the aircraft which, of course, would cause the shock absorbers 2| to yieldto a greater or lesser extent. 4

In, the preferred arrangement, the latches will be located at such elevations that latch engage-j ment with the grillwork will occur within the upper and lower limits of the normal shock absorption travel of the wheels,

The mechanism, described may further be em.. ployed for take-01f purposes under adverse. con--. ditions. Thus, with an aircraft equ pped with. means for effecting jump take-01f, as above men-, tioned, after driving the rotor toa high speed with the blade pitch set at zero, the pilot, mayv simultaneously disconnect the rotor drive clutc (H112) and increase the blade pitch, andat the. same time retract the latches 3| on thestrut. In this way the machine is held to the platform; until the moment when take-off is desired.-

Interlocked controls for disconnecting the rotor. driving clutch and increasing the blade pitch may be employed in accordance with copending applie; cation of Agnew E. Larsen, Serial No. 363,593, filed October 31, 1940, which application issued July 20, 1943, as Patent No. 2,324,588. Thus, by way of illustration, a control valve 49 may .be employed to deliver fluid under pressure from supply line 49a to the pipe 230. which extends to the blade pitch control cylinders 23, andalso to additional piping Hlc which is extended to a suitable. actuating cylinder and piston device (not shown) for engaging the rotor drive clutch. Re-- lease of pressure from piping 23a and Inc may be effected by movement of the control valve 49 so as to connect the piping with the release line 49b, whereupon increase of blade pitch and.dis-,

connection of the rotor. drive is brought about, in accordance with the aforementioned Ray patent and Larsen application. The supply and release lines 49a and 492) may be coupled with any suitable fluid pressure system such as that disclosed in said Larsen application 363,593, (now Patent No. 2,324,588). 1

Although, as above indicated, the latches 3-l of the hold-down strut may be employed-to retain the craft in proper position during driving of the rotor in preparation for take-off, an independent hold-down means is shown for this purpose in Figure 1, it being preferable to employ'the independent means for reasons which will further appear.

Such independent means (see Figures 1 and 5) may comprise a cable 15 anchored to the platform 24 and having an eye 16 at its upper end adapted to be engaged by the releasable hook H which is pivotally mounted on bar 18 suspended by a jointed link I9 from a fixed part on the fuselage framing, as indicated at 85. A releasable catch 8!, pivoted on bar 18, is adapted to engage the hook TI and retain the same in closed position under the influence of a spring 82. Catch 8| may be pivoted upwardly to release the book. by a cable 83, the upper end of which issecured to stem 84 projecting from piston 85 which works in cylinder 86. The piston 85, and thus the catch 8!, is normally urged to the upper position by a spring 81, and the piston may be moved to the lower end of the cylinder by admission of fluid pressure to the cylinder through connection 88 which, as shown in Figure 1, is coupled .with piping 100, to receive fluid under pressure therefrom when thecontrol valve 49 is adjusted to engage tli'e rotor driveclutch and reduce the blade pitch. -'I"hus,' upon' engagement of the rotor drive clutch to initiate rotation of the rotorin preparationfor take-off, the catch 8 is retained in operative position by spring 82, thereby holding the hook in closed-position so as to hold the aircrafttightly to the platform by the cable 153 until take-off is desired. Uponactuation of the control valve to-disconnectthe rotor drive clutch and increase the blade pitch for take-01f, the pressure released at the same time from cylinder 86-permits spring 81 to draw the catch 8| upwardly, thereby freeing hook 11' so that upon take-. off the eye-1.6 may freely slide off the hook and notinterfere with take-off.

=Th'e mechanisms provided in accordance with Figures 1 to inclusive are'adapted to meet widel-y varying landing and take-off condition. The safety otoperation-of an aircraftto and from a small landing platform on board ship is thereby greatly enhanced, particularly in view of the fact that both the landing and the take-off hold-down devices are arranged for automatic operation,

- In the modification of Figure 6, it is contemplated that the strut be mounted in the same manneras that hereinbefore described, and further that the strut be equipped with the same type of retractable latches 31, with operating cables an'd ring and 36, as before.

Here, however, the lower member 28a of the strut hasa piston at its upper end working a. cylinder 5| carried by the upper member 29:: of the strut. Acompression spring 52 normally urges the lower member 28a to its lower position. Pressure fluid is adapted to be admitted to the underside of piston 56 through port 53,.

whichis coupled by means of piping 54 and one or more flexible tubes 55 with a valve casing .56. Branches 54a and 54b connectv piping 55 with angularly spaced valve ports 51 and 58 arranged to be connected with ports 59 and. 60, respectively,

by means of transverse port 6| in a rotatable valve 62. Fluid under pressure is delivered to port .59' through connection 63 from pump 64, which latter derives fluid from reservoir 65 through pipe 66. A return or relief line 6! couples port Gil with the reservoir; The valve 62 is actuable by an arm 68 which is adapted to be connected through link 69 with a shaft 10 or some other part which partakes of the vertical shock absorption. movement of landing wheel it.

When the aircraft is in flight, the parts occupy the positions shown in full lines in Figure 6, at which time pressure from cylinder 5| is exhausted through piping 54, branch 54b, valve passage 6 I, and pipe 61 to the fluid reservoir. Upon making a landing, the upward shock absorption movementtcf wheel l9 moves valve 62 so as to couple branch 54a with the pressure line 63, thereby delivering fluid through piping 5:3 to the bottom of cylinder 5| which, in turn, forces strut member 280 upwardly, thus serving to place the strut under tension to hold the aircraft tightly to the landing platform. It will be understood that the length of the strut, or its projection thereof downwardly from the body of the aircraft, is such that the latches 3| will engage grillwork elements of the landing platform prior to appreciable upward shock absorption movement of the landing wheels l9, thereby ensuring engagement of the latches before the fluid pressure system serves to contract the strut.

.It is contemplated that a readily removable pin ll be employed for the connection between link 69 and. valve operating arm 63, so that after a landing has been made, an attendant may readily withdraw the pin and then manually move the valve operating arm 63 to release the pressure from cylinder 5 I, after which the lower strut member 23a may be withdrawn, and the aircraft moved about, as desired.

Preferably, although not essentially, the arrangement of Figure 6 further includes a spring pressed releasable latch 12 adapted to engage ratchet teeth 73 formed at one side of the upper extension 28?) of. the strut member 2811. This device will serve to retain the barbs 3| in engagement with the landing platform even after exhaust of fluid pressure from the lower end of cylinder 5|.

In the modification shown in Figures 7, 8 and 9, provision is made for automatic release of the barbs on the hold-down strut when the controls are actuated to effect direct take-off. Thus, in this form, a single hold-down means may be employed not only for landing, but also for takeoff.

In Figure '2, the upper end of the hold-down strut appears at 29, the barbs (not shown in this view) being retractable by upward movement of element 33, as in Figure I. Said movement may be effected by means of a lever 89 pivoted at 98 to a bracket 9i which is mounted to move with the hold-down strut. The free end of lever 69 is pinned at 82 to a cylinder 93, and piston 94 working in cylinder has a stem 35 which projectg for connection at with a part of bracket ti. Admission of pressure fluid to the upper end of cylinder @3 through piping 9'? (which may incorporate flexible tubing) causes lever 89 to move upwardly and thus retract the barbs at the lower end of the strut.

Piping 9? is associated with a control system including a control valve 98, which serves also for engagement and disengagement of the rotor clutch and for adjustment of the blade pitch. In a manner similar to the control system of Figure 1, pressure may be brought to valve 93 through connection 69a, and the branched pipe 491) may serve for pressure release. Branches I00 and 23a of pipe extend, respectively, to the rotor drive clutch and the blade pitch control cylinders 23. As clearly appears from the showing of the valve in Figure '7, in one position of adjustment, the valve member ifiil serves to couple the pressure supply line lsa with pipes its and 23a so as to engage the rotor clutch and reduce the blade pitch, for drive of the rotor in preparation for take-off. At the same time, valve member I00 interconnects piping 91 with one branch of the release line 49b; so that the pressure is exhausted from cylinder 93 and the barbs iii of the holddown strut may remain extended and in engagement with the landing platform.

When the valve iiiil is moved to the position shown in Figure 8, pressure is released from pipes li e and met-through pipe 99 and to the relief line see, thereby disengaging the rotor drive clutch and permitting the blade pitch to increase for take-off. In this adjustment, moreover, the pressure supply line 49a is coupled with piping 91 extended to cylinder 93, so that fluid pressure is admitted to cylinder 93 to move the barb control element 36 upwardly and thus retract the barbs to permit take-oh.

Valve its is capablev of another adjustment, as shown in Figure 9, in which the pressure supply line 49a, is blocked and all of pipes Inc, 23a and p9 91 areconnected with therelease line 491). With this-setting, the barbs are permitted to. be extended, the rotor clutch is disconnected, and the blade-pitch is retained at the normal flight setting, so that the hold-down strut is conditioned for cooperation with the platform in making a landing.

A very important advantage of the present invention is that the required accuracy of spotting in making a landing is minimized,'since the hold-down device will be effective at any point over the area of the landing platform.

I claim:

1. In an aircraft capable of substantially vertical descent to a landing and having landing elements supported from the craft with freedom for shock absorption movement in a generally vertical direction, a device carried by the aircraft for holding the aircraft down upon making a landing, said device comprising a strut extending downwardly from the aircraft to project into an aperture of a grillwork landing platform and incorporating relatively movable parts to provide for extension and contraction of the strut, a lower part of said strut having means for engagement with a grillwork element, and means automatically operative upon making a landing to contract said strut, thus drawing the aircraft tightly to the landing platform.

2. A construction in accordance with claim 1 i in which said last means is associated with a landing element of the aircraft and is operated under the influence of shock absorption movement of said element.

3. A construction in accordance with claim 1 and further including releasable latch means operative to prevent extension of said strut until release thereof.

4. In an aircraft capable of substantially vertical descent to a landing and having landing elements supported from the craft with freed0m for shock absorption movement in a generally vertical direction, a device carried by the aircraft for holding the aircraft down upon making a landing, said device including a member adapted to project through an aperture of a grillwork landing platform and having a retractable latch or barb adapted to engage an under surface of a grillwork element, the parts being so relatively disposed that said latch will descend sufficiently to engage a grillwork element at least within the limits of said shock absorption movement of the landing elements of the aircraft.

5. A construction in accordance with claim 4 and further including means automatically operative upon making a landing to draw said member with the latch carried thereby toward the aircraft, thus tightening the latch against the grillwork element engaged thereby.

6. In an aircraft capable of substantially vertical descent to a landing and having landing elements supported from the craft with freedom for shock absorption movement in a generally vertical direction, a device carried by the aircraft for holding the aircraft down upon making a landing, said device comprising a strut extending downwardly from the aircraft to project into an aperture of a grillwork landing platform, and a plurality of vertically spaced retractable barbs carried by said strut adapted to engage grillwork of the landing platform.

'7. In combination with an aircraft capable of substantially vertical ascent and descent, and with a landing platform, mechanism associated iii) with said platform and said aircraft comprising complementary releasable interlocking hold down elements adapted to interengage and hold the craft to the platform in any one" of a pluralityof different positions on the platform, means placing said elements under .appr'oxi ma'tely vertical tension when interengaged, and pilot operated means for releasing the hold down elements 'whenit is desiredto effect stantiallyyertical take-off.

8. In combination with a retaining grillwork, and an aircraft having a sustaining rotor providing for generally vertical landing and take-off, a device carried by the aircraft for holding the aircraft down upon making a landing, said device comprising a member adapted to project into a grillwork aperture and having means automatically engaging a grillwork element when the landing is effected, additional hold-down means for the aircraft operative to permit ready release of said device after landing and comprising a releasable tie, and means operable by the pilot for releasing said tie when it is desired to effect generally vertical take-off.

9. A construction in accordance with claim 8 in which the aircraft further incorporates a primary control for effecting vertical take-off, and mechanism interrelating the operation of the releasable tie and said primary control and providing for release of the tie upon actuation of the control to effect take-off.

10. In an aircraft capable of substantially vertical descent to a landing and having landing elements supported from the craft with freedom for shock absorption movement in a generally vertical direction, a device carried by the aircraft for holding the aircraft down upon making a landing, said device comprising a strut extending downwardly from the aircraft to project into an aperture of a grillwork landing platform and incorporating relatively movable parts to provide for extension and contraction of the strut, a lower part of said strut having means for engagement with a grillwork element, and releasable means for normally retaining said relatively movable parts of the strut in a predetermined position intermediate the limits of movement, to provide for disengagement of the strut from the grillwork platform after a landing had been made. ,7

11. In aircraft hold-down eq ipment,- a downwardly-projecting hold-down device on the aircraft and retaining grid adapted for cooperation with the hold-down device on said downwardly-projecting aircraft, comprising grid elements which in cross-section taper to present a narrowed upper edge so as to facilitate guiding of the hold-down device into the spaces between grid elements.

12. A construction according to claim 11 wherein the grid elements are of extended vertical depth so as to have greater inherent stiffness than a wire-type grid.

13. In an aircraft capable of substantially vertical descent to a landing platform and having landing elements, a device carried by the aircraft 1 adapted to project into an aperture of a grillwork and having means automatically engaging the under side of such grillwork in position to place themeniber under direct vertical tension within the range'of shock absorption movement of the landing elements upon completion of a landing, thus holding the aircraft tightly to the landing platform.

14. For useyvith an aircraft having a compres- 12 locking elements associated respectively with the craft and with an aircraft supporting base, the interlocking element associated with the aircraft 'being connected with the aircraft independently of the compressible landing gear so as to move downwardly with the aircraft during descent to a landing and thereby provide for holding the landing gear under. a compression greater than the dead weight of the craft by reaction between sible landing-gear, an aircraft hold-down device 10 said craft and said supporting base.

having "complementary automatically inter- .HAROLD F. PITCAIRN. 

